1. Field of the Invention
This invention relates to the use of a catalyst composition in the catalytic conversion of CO to CO.sub.2. More particularly the invention is concerned with a process for reducing the CO level of effluent gases from the regeneration of a cracking catalyst by contacting them with a spent noble metal reforming catalyst.
2. Description of the Prior Art
Various processes such as cracking, hydrocracking, etc. are known for the conversion of hydrocarbons to lower molecular weight products. The catalytic cracking of heavy petroleum fractions is one of the major refining operations employed in the conversion of crude petroleum oils to desirable fuel products such as heating oils and high octane gasoline. Illustrative of "fluid" catalytic conversion processes is the fluid catalytic cracking process wherein suitable preheated high molecular weight hydrocarbon liquids and vapors are contacted with hot finely divided, solid catalyst particles, either in a fluidized bed reactor or in an elongated riser reactor and maintained at an elevated temperature in the fluidized or dispersed state for a period of time sufficient to effect the desired degree of cracking to lower molecular weight hydrocarbons.
In the catalytic process some non-volatile carbonaceous material or "coke" is deposited on the catalyst particles. As coke builds up on the catalyst, the activity of the catalyst for cracking and the selectivity of the catalyst for producing desirable products diminish. The catalyst particles may recover a major proportion of their original activity by removal of most of the coke by a suitable regeneration process. The catalyst regeneration is accomplished by burning the coke deposits from the catalyst surface with an oxygen-containing gas such as air. Many regeneration techniques are practiced commercially whereby a significant restoration of catalyst activity is achieved. The burning of coke deposits from the catalysts requires a large volume of oxygen or air and produces substantial quantities of CO and CO.sub.2. Ordinarily the regeneration is conducted at a temperature ranging from about 1050.degree. to about 1250.degree. F. The effect of any increase in temperature is reflected in an increased rate of combustion of carbon and the more complete removal of carbon or coke from the catalyst particles.
A major problem often encountered in the practice of fluid catalyst regeneration is the phenomenon known as "after burning" which is descriptive of the further combustion of CO to CO.sub.2. The operators of fluid catalyst regenerators avoid after burning because it could lead to very high temperatures which are damaging to equipment and possibly to the catalyst particles.
More recently, as operators have sought to raise regenerator temperatures for various reasons, elaborate arrangements have also been developed for control of regenerator temperatures at the point of incipient after burning by suitable means for control of the oxygen supplied to the regenerator. However, with the control of after burning the flue gas from catalyst regenerators usually contains very little oxygen and a substantial quantity of CO and CO.sub.2. In order to substantially eliminate the CO from the flue gas and to recover heat energy from the combustion of CO to CO.sub.2, the regenerator flue gas is generally sent to a CO boiler wherein the combustion of CO is performed.
There has appeared in the literature, e.g., U.S. Pat. Nos. 2,382,382 and 3,563,911, various techniques for substantially eliminating both uncontrolled after burning and the presence of CO in the regenerator effluent flue gas. These techniques generally involve the use of relatively high regeneration temperatures, e.g., 1275.degree. to 1400.degree. F., and the presence of relatively high concentrations of O.sub.2 in the regenerator so that there is substantially complete combustion of the spent catalyst coke to CO.sub.2 in the regeneration vessel.
It has also been disclosed in the literature, e.g., U.S. Pat. Nos. 2,414,002, 2,436,927, 2,647,860, 3,650,990, 3,788,977 and Netherlands Pat. No. 7,412,423, that the presence of CO in the regenerator effluent gas from a catalytic cracking operation can be substantially reduced by incorporating a small amount of CO oxidation promoter in the cracking catalyst composition. For example, cracking catalyst composites consisting of a crystalline aluminosilicate zeolite in a silica-alumina matrix and a small amount of platinum or palladium are known to yield relatively small amounts of CO in the flue gas during the regeneration thereof.
In accordance with the present invention, it has been found that small amounts of spent noble metal reforming catalyst situated downstream from a cracking catalyst regeneration zone can be effectively utilized to catalyze the combustion of CO to CO.sub.2. This will provide a new use for a spent catalyst which otherwise would have no catalytic use. The invention also eliminates the potential deleterious catalytic effects of the noble metals in a cracking environment (e.g., high coke means, high H.sub.2 yields, etc.). Further, the noble metal on the spent noble metal reforming catalyst used in the present invention as a CO oxidation promoter can be recovered chemically and re-used. When dispersed on the cracking catalyst as in prior art disclosures, the noble metal cannot be recovered for re-use economically.